The present invention relates to optical Bragg-reflectors, and more particularly to the alteration of the reflective properties of optical Bragg-reflectors.
The need for capacity in communications networks is increasing exponentially, and presently capacity demands are doubling every eighteen months or even less. Efforts are being made in the telecom and computer business to meet this incredible expansion of bandwidth requirements.
One obvious action is to simply draw new optical cables between nodes of the network. However, this approach is costly and hardly a successful way of meeting the upcoming demands. Instead, measures are focused, when possible, on augmenting the capacity of existing fiber networks.
One widespread method of achieving increased transfer capacity in existing fiber networks is wavelength division multiplexing (WDM). In WDM, a single optical fiber is used for the transmission of several channels of information, each channel being associated with a specific wavelength. For a background of this technology, the reader is directed to, xe2x80x9cA review of WDM technology and applicationsxe2x80x9d, Opt. Fiber Technol., 5, pp. 3-39 (1999).
While the use of WDM in optical networks yields a substantial increase in transfer capacity, the complexity of the communications systems increases correspondingly. Furthermore, the number of physical components in each link of the network increases, the requirements on each component increase, and detecting and localizing errors or imperfections in the network is rendered more difficult.
The optical networks of today mainly use point-to-point communication, wherein electronically encoded information in one node of the network is transformed into optically encoded information, and then transferred, through a sequence of optical fiber amplifiers and optical transport fibers, to another node, where the optically encoded information is transformed back into electrically encoded information.
However, this prior art is associated with major drawbacks and limitations.
If, for instance, an error occurs in one of the fiber amplifiers, it is very hard to determine which amplifier, in the sequence of amplifiers, is the faulty one.
Furthermore, technology is being pushed towards communications systems in which the optical signal is passed via all-optical switching nodes, where the signal is routed without any need for optical-to-electrical conversion or vice versa. Thus, if an error is detected in a receiving node of the communications network, it is extremely hard to localize the origin of error.
If the light signal propagating in an optical transport fiber could be analyzed in an accurate and simple fashion, without interfering with the signal, any flaws in the optical communications network could be detected at an early stage. In the prior art, such analysis has been too complicated, too expensive and too inaccurate to gain widespread acceptance.
Obviously, it is of utmost importance to be able to monitor the power spectrum of the light propagating in the optical transport fiber.
It is known in the prior art to use a phase grating in an optical fiber in order to filter out one desired wavelength from a broadband light signal. A phase grating will reflect one predefined wavelength and leave other wavelengths essentially undisturbed.
An optical phase grating is a structure of essentially periodically varying refractive index in an optically transparent medium. An overview of the technology is given in M. C. Hutley, xe2x80x9cDiffraction gratingsxe2x80x9d, Academic Press, London (1982). When light is incident on an optical phase grating, a small part of the incident light is reflected off each grating element (period). When a multiplicity of grating elements are arranged after each other (i.e., arranged as a phase grating), the total reflected light would be the sum of all these single reflections. The fraction of the incident light reflected off each grating element is determined by the depth (amplitude) of the refractive index modulation in the phase grating. The deeper the modulation, the larger fraction of the incident light is reflected off each grating element. If the incident light is essentially normal to the grating (i.e., to the grating elements), the grating is said to act in the Bragg domain and is named xe2x80x9cBragg gratingxe2x80x9d. Light reflected off each grating element will thus overlap the light reflected off the other elements, causing interference. For a certain wavelength, all these reflections are in phase, whereby constructive interference is effected. Although each reflection is small, a substantial reflection is obtained due to constructive interference. The wavelength for which constructive interference is effected is called the xe2x80x9cBragg wavelength,xe2x80x9d xcexBragg, and is given by (at normal angle of incidence)
xcexbragg=2nxcex9
where n is the average of the refractive index, and xcex9 is the grating period.
If the grating period, xcex9, varies along the grating, the grating is said to be a xe2x80x9cchirpedxe2x80x9d Bragg grating. In a chirped Bragg grating, different wavelengths are reflected in different portions of the grating, in fact, making the Bragg grating a broadband reflector, a chirped Bragg-reflector.
U.S. Pat. No. 6,052,179 discloses a system for determining the average wavelength of light transmitted through an optical fiber. In this system, a chirped Bragg grating is provided in an optical fiber, the grating having a modulation amplitude that varies from a first end to a second end of the fiber grating. Thus, the reflectivity at one wavelength is different from the reflectivity at another wavelength, since different wavelengths are reflected at different portions of the grating. Based on the output from two photo detectors, one detecting the light transmitted through the grating and the other acting as a reference, the average wavelength of light transmitted through the grating is determined. However, this approach has several disadvantages. Firstly, the light to be analyzed has to be divided into two separate fibers, which is a serious complication in itself. Secondly, only the average wavelength can be determined. There is no possibility for proper spectrum analysis by the system disclosed in the above reference. Furthermore, the system is intended for sensing applications, where external influence changes the average wavelength coming into the system.
Previous attempts to alter the reflective properties of an optical Bragg grating include creation of an acousto-optic superlattice imposed on a chirped Bragg grating. Such an arrangement is described by Chen et al. in xe2x80x9cSuperchirped moirxc3xa9grating based on an acousto-optic superlattice with a chirped fiber Bragg gratingxe2x80x9d, Optics Letters, Vol. 24, No. 22, pp. 1558-1560. According to this reference, multiple transmission peaks are obtained by superimposing an acoustic wave on a chirped Bragg grating. The spacing of the transmission peaks is varied by the acoustic frequency. However, this arrangement does not permit transmission of one single wavelength only, and the variation of the spacing of the transmission peaks by acoustic frequency is very limited.
The present invention provides new methods and arrangements for establishing transmission of light through a reflecting Bragg grating, and for utilizing such transmission for analysis of the characteristics of a light signal. The drawbacks and limitations associated with the prior art are effectively eliminated by a method and an arrangement of the general kind set forth in the accompanying claims.
The present invention has further advantages, which will be apparent from the detailed description set forth below.
It is a general object of the present invention to provide a method of establishing transmission of light through a broadband, chirped Bragg-reflector. The method can be used for analysis of the power spectrum of a light signal, as well as for other applications where it is desirable to transmit a certain wavelength component of light through a structure at a certain instant in time.
Furthermore, it is an object of the present invention to provide a method and an arrangement for spectrum analysis of a light signal, which method and arrangement essentially eliminate the aforementioned drawbacks and limitations of the prior art. Briefly stated, this is obtained by establishing transmission of light through a chirped Bragg-reflector by means of a longitudinal acoustic pulse being present in the grating structure. The presence of the acoustic pulse alters the reflective properties of the Bragg-reflector, thus giving rise to transmission of a specific wavelength of light for each position of the acoustic pulse in the Bragg-reflector.
According to a first aspect of the present invention, a method of establishing transmission of light through a chirped Bragg-reflector is provided, by which method a certain wavelength component is transmitted through the Bragg-reflector at a certain (corresponding) instant in time. In an unperturbed state, the Bragg-reflector is reflecting essentially all incident light within a predefined wavelength range. According to the invention, light is incident into an optical waveguide incorporating a chirped Bragg-reflector. The reflective properties of said Bragg-reflector are altered by sending a longitudinal acoustic pulse into said waveguide for propagation along the same. For each location of said acoustic pulse in the chirped Bragg-reflector, the reflectivity for a wavelength associated with said location in said Bragg-reflector is altered, thereby establishing transmission of the wavelength at issue.
According to a second aspect of the present invention, a new method of analyzing the power spectrum of a light signal is provided. The method of analyzing the power spectrum is based on the aforementioned method of establishing transmission of light through a chirped Bragg-reflector. Briefly stated, analysis of the power spectrum is obtained by monitoring the light transmitted through the Bragg-reflector, and by subsequent analysis of the monitored signal.
One advantage of this method is that it is sufficient to tap off only a small part (typically about 1% or less) of the light propagating in an optical transport fiber into a secondary fiber. The light in the secondary fiber is then analyzed, and the interfering effect on the transport fiber is negligible. The light is tapped off from the transport fiber by non-wavelength discriminating coupling means, as known in the art. By analyzing the light in the secondary fiber by the method according to the present invention, the power spectrum of the light signal propagating in said transport fiber is determined with very high accuracy. Furthermore, the light signal in the transport fiber is essentially undisturbed, apart from the 1% tapped off.
Another advantage of the present invention is that it also allows for real time supervision of a fiber-based communications system. The information obtained from such supervision can advantageously be utilized for controlling other equipment connected to said system, such as amplifiers, filters, etc. In-line amplifiers and filters can be controlled in a feedback configuration with a spectrum analyzer according to the present invention.
Yet another advantage of the present invention is that a wavelength scan of transmitted light is obtained, which is very convenient for spectrum analysis. The present invention can provide for repeated wavelength scans, thereby facilitating interpretation and analysis of the monitored signal.
According to a third aspect of the present invention, an arrangement is provided for measuring and characterizing the power spectrum of a light signal propagating in an optical fiber. Such an arrangement includes a chirped Bragg-reflector operating in accordance with the methods above.
The present invention is based on the general insight that the reflective properties of a chirped Bragg grating (or chirped Bragg-reflector) can be altered by a longitudinal acoustic pulse propagating along the grating. More particularly, the present invention is further based on the deeper insight that a longitudinal acoustic pulse of carefully chosen shape propagating along the grating can give highly accurate, and wavelength separated in time, transmission through the chirped Bragg grating.
According to one embodiment of the present invention, a method of establishing transmission of light through a chirped Bragg-reflector includes the steps of directing light into a light guiding structure provided with said Bragg-reflector, and sending a longitudinal acoustic pulse along said light guiding structure, thereby locally and temporarily altering the reflective properties of the Bragg-reflector for a certain wavelength corresponding to the momentary position of the travelling acoustic pulse.
According to another embodiment of the present invention, an optical fiber is provided with a chirped Bragg grating. The fiber has an input end and an output end, for the input of a light signal into said fiber and for the output of a transmitted part of said light signal, respectively. Connected to the optical fiber is an acoustic actuator for the emission of a longitudinal acoustic pulse into said fiber. The acoustic pulse is given a shape that has the effect of lowering the reflectance of the chirped Bragg-reflector for a certain wavelength at a certain instant in time. Preferably, the acoustic pulse is given an anti-symmetric shape, whereby an etalon effect is achieved in the proximity of the travelling acoustic pulse, thereby locally lowering the reflectivity of the chirped Bragg-reflector to essentially zero.